Rough Idle in Drive (or reverse) with 1993 1FZ

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I just got my 1993 land cruiser running again after doing the head gasket repair. In park and neutral, the idle is fine (~700-800 rpm when warm). When I shift into reverse or drive, the idle falls to ~400 and often time stalls out. It's almost like the ECM isn't recognizing or compensating for the extra load of being in drive/reverse. There is no check engine light once the engine is running.

I couldn't find anything in the FSM about this specific scenario so I started searching this forum also have not found an exact match to my issue. If I missed someone's thread on a similar issue, I'd love to read through that. There was some mention of the "PNP switch circuit" in the FSM troubleshooting table but I'm not familiar with what the PNP switch is (shifting switch?).

I do have some exhaust leaks in my downpipes and Cat's but dropping ~$1500-$2000 on downpipes and CA legal Cat's isn't in the budget right now.

Here is everything I replaced while doing the head
1. all ignition things (coil, wires, cap, rotor, plugs)
2. all fuel injectors
3. all vac lines
4. fuel filter

Some things I'm going to try (but please add other ideas)
1. IAC cleaning (I measured the resistance but will try cleaning it per a success in a different thread)
2. Double check ignition timing
3. Double check my compression test
4. Change trans fluid though it is fresh ~3k miles ago
5. Re-check VAF and TPS despite testing them a week ago for a different reason.
 
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In park and neutral, the idle is fine (~700-800 rpm when warm)
That's not fine. Idle should be 650 +/- 50 RPM.

How long has the vehicle run since doing the work? Presumably, the battery was disconnected and the ECU has to re-learn.

You say that you replaced all of the vacuum lines, but did you verify that the internal passages are clear? Are you certain that you connected everything correctly on the underside of the intake? Did you test your EGR modulator?

Did you verify IAC operation? You just need to apply voltage and verify that it "ticks" (moves a little bit each time voltage is applied).
 

BigAxeJack

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I've heard of these symptoms occurring with a cracked intake hose. It's the long rubber/plastic "pipe" from the air filter to the throttle body. Worth a peek anyway. Good Luck!
 
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That's not fine. Idle should be 650 +/- 50 RPM.

How long has the vehicle run since doing the work? Presumably, the battery was disconnected and the ECU has to re-learn.

You say that you replaced all of the vacuum lines, but did you verify that the internal passages are clear? Are you certain that you connected everything correctly on the underside of the intake? Did you test your EGR modulator?

Did you verify IAC operation? You just need to apply voltage and verify that it "ticks" (moves a little bit each time voltage is applied).

Hmmm. Thanks for pointing out the idle issue. I'll check my accelerator cable adjustment again to make sure that isn't causing it.

It's run for about 45 min now since the work was completed. It definitely went through the re-learning process with rough idle all the time for the first couple thermal cycles.

As far as internal passages, are you referring to the hard lines and ports in the intake manifold? If so, I did not specifically blow through the hardlines but I cleaned the intake manifold ports. I did follow the FSM for the vac line replacements and replaced them one at a time to not get things mixed up. I have not checked the EGR modulator. I'll look for that test in the FSM.

I only checked the resistance of the IAC but will be trying the applied voltage test.

Thanks for the response and ideas!
 
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As far as internal passages, are you referring to the hard lines and ports in the intake manifold? If so, I did not specifically blow through the hardlines but I cleaned the intake manifold ports.

Yep. Along with the passages for stuff like the IAC. Mine was packed with carbon everywhere, but if yours wasn't obviously in need of cleaning then it might not be an issue for you. Someone once posted here about running steel guitar string through to verify that the lines through the intake are clear.

One other thing: the vacuum line that I had forgotten about was the one for the brake booster. That finally fixed the last of my idle issues.
 
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Yep. Along with the passages for stuff like the IAC. Mine was packed with carbon everywhere, but if yours wasn't obviously in need of cleaning then it might not be an issue for you. Someone once posted here about running steel guitar string through to verify that the lines through the intake are clear.

One other thing: the vacuum line that I had forgotten about was the one for the brake booster. That finally fixed the last of my idle issues.
Ok great, I'll make sure to check that those hardlines and what not are clear and that the IAC is making sound with voltage applied. My EGR ports in the intake manifold were packed so I bet that the hardlines and ports may need more attention as well.

I actually did the brake booster line last night, so I'm not as lucky :)
 
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I've heard of these symptoms occurring with a cracked intake hose. It's the long rubber/plastic "pipe" from the air filter to the throttle body. Worth a peek anyway. Good Luck!
Good point, I had seen that too. I need to search/figure out the best method for confirming that it is not leaking but I'll definitely check it out.
 

BigAxeJack

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Good point, I had seen that too. I need to search/figure out the best method for confirming that it is not leaking but I'll definitely check it out.
You can usually see the cracks, typically in the "valleys" between the accordion folds. I've seen it fixed with duct tape, but the long term solution is a replacement. It would make sense that this happened during your major project, because the pipe gets damaged when removed.
 

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